The anatomy of the inferior-collicular complex of the barn owl, situated below the fourth ventricle in the tectal lobe, was studied by determining the distribution of antigens with antibodies directed against tyrosine hydroxylase, gamma-aminobutyric acid (GABA)(Abeta), dopamine- and cyclic AMP-regulated phosphoprotein (DARPP-32), calretinin, and calbindin. Additionally, the somata were stained with cresyl violet, and fibers were marked according to the Gallyas procedure. These markers were chosen to allow for an easy delineation of the boundaries between the subnuclei of the inferior colliculus. We could discriminate eight structures that belong to the three subnuclei of the inferior colliculus [the central nucleus (ICC), the superficial nucleus (ICS), the external nucleus (ICX)] and to the optic tectum. Periventricular tectal layers 15a and 15b stained well with all the antibodies used. The ICS, embedded in tectal layer 15a, may be divided into a dorsal and a ventral lamina. It does not have direct contact with the other nuclei of the inferior colliculus. The border between tectal layer 15a and ICX was well marked by all antibodies, but less so in Gallyas and cresyl violet stains. The ICC consists of a core and a medial and lateral shell. The core was clearly demarcated with antibodies against calretinin and calbindin. The border between the lateral shell and the ICX was marked less well than the borders between ICX and 15a, but the somata were much more darkly labeled with the DARPP-32 antibody in ICX than in the lateral shell of ICC. None of the markers delineated the border between the medial and lateral shell of ICC.
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